Where is texture mapping panel in 2.8?
The Blender Internal (BI) render engine is no longer a part of Blender since the 2.80 release, which means that this specific panel no longer exists. However Cycles and Eevee both have a node-based material system that lets you configure the same properties. Switch to the Shading workspace or open the Shader Editor in one of the areas.


Shading Workspace


Shader Editor

The basic node setup uses a Texture Coordinate, 
Mapping and Image Texture node.



The Texture Coordinate node provides access to the active UV map, which can be used as input to the Mapping node. Similar to the texture panel for Blender Internal, you can adjust the location (BI: Offset) and scale (BI: Size) as well as the rotation. The vector output of the mapping node is connected to the Image Texture node. There you can find the Projection property. 

The UV Map node is useful if you want to use a specific UV map, not just the currently active one. In this case you can replace the Texture Coordinate node with it.

All nodes offer even more options, that are documented in the linked pages of Blender's manual.